The invention extends the prior concepts of a multi-tone photonic oscillator and of a single-tone opto-electronic oscillator to multiple optical wavelengths. The prior approaches supply only a single optical wavelength. Using these prior approaches, a multiple wavelength output would require multiple oscillators, with each oscillator operating at a different optical wavelength. If those oscillators have RF tones of nominally the same frequency modulated onto the optical carrier, the RF signals produced at photodetectors connected to the outputs of those oscillators would have the same frequency. However, those RF signals would not have any phase coherence. There is a need for the generation of phase-coherent RF tones on optical carriers having different wavelengths.
Prior art photonic oscillators (also called opto-electronic oscillators) are described in U.S. Pat. Nos. 5,723,856 and 5,777,778 by Yao and Maleki of JPL. The photonic oscillator of these prior inventions comprises an electro-optic modulator, a photodetector and a feedback loop that goes from the optical output of the modulator into the optical input of the photodetector, then electrically out of the photodetector and into the electrical drive input of the modulator; with the loop gain being greater than one. An optical source, such as a pump laser, supplies the light that goes into the modulator. Part of the light output from the modulator is diverted into the feedback loop.
A photonic oscillator that applies a multi-tone RF modulation on a single wavelength optical carrier without using a separate electronic RF oscillator is disclosed in PCT patent application Ser. No. PCT/US02/36849 filed Nov. 15, 2002 by HRL Laboratories, Yap and Sayyah entitled “Agile Spread Waveform Generator and Photonic Oscillator” which application is owned by the assignee of the present application.
These prior inventions do not make any assertions regarding the wavelength or wavelengths of light provided by the optical source. The optical wavelength of the light is not a characteristic that is a point of concern in these prior inventions and no mention is made of the wavelength. Also, no distinction is made between the wavelength or wavelengths of the light that is diverted into the loop compared to those not diverted into the feedback loop. What is considered important in these prior inventions, instead, is the optical power of that light, which, if sufficiently high, can supply the necessary energy to sustain the oscillation.
These prior photonic oscillators do not specifically produce optical outputs that have multiple wavelengths nor are their output wavelengths necessarily suitable for distribution of the light by a WDM network. The multi-wavelength capability of the present invention is one feature that is of benefit to the art.
Digital beam forming is being developed for phased array antennas that have multiple beams. Typically, a different digital beam forming signal is generated for each antenna element. This signal can contain the data to be transmitted and also the phase information for forming the antenna beam. This signal is then frequency-converted to the desired RF carrier and supplied to the antenna element. As illustrated in FIG. 6, optical links can be used to carry the data from a central processor, which generates the beam forming information, to the antenna. An optical heterodyning technique can be used to also accomplish the frequency conversion as part of the optical distribution of the data. Alternatively, a cascade of two electro-optic modulation units can accomplish the frequency conversion. Since different signals are needed for each antenna element, many optical links would be needed, one for each antenna element. To reduce the number of optical fibers required, it may be advantageous to use a WDM network to carry signals between the processor and the antenna. A single optical fiber could carry the signals for a subarray of multiple elements. Each element of the subarray would be distinguished by a different optical wavelength. These multiple optical wavelengths would be carried in the same fiber.